Hydraulically controlled grader-leveler adapted for trailing by tractors

ABSTRACT

A chassis, adapted to be connected to a farm tractor, supports a blade for grading and levelling; the blade is rotatable with respect to the chassis. The angle of the blade is set by a hydraulic transmission, such as a linear or rotary hydraulic motor controlled by a direction controlled valve. Mechanical output from the hydraulic motor is connected to the blade support, typically a turntable, to set the blade angle as determined by the application of hydraulic pressure. The blade can be fixed in position by engaging tooth-and-detent elements, one of them being respectively fixed to the chassis and the other formed on a turntable which is rotatable with the blade.

United States Patent Manor 145} Dec. 9, 1975 [5 HYDRAULICALLY CONTROLLED1,844,058 2/1932 Dean 172/796 GRADER LEVELER ADAPTED FDR 2,720,71610/1955 White, Jr... 37/DIG. 20 3,716,105 2/1973 Hallam 172/781 xTRAILING BY TRACTORS 3,736,845 6/1973 Weiste 91/390 Inventor: GedalyahuManor, Haifa, Israel Assignees: Technion Research and DevelopmentFoundation, Ltd.; Aharon Kipnis, both of Haifa, Israel Filed: Dec. 20,1973 Appl. No.: 426,522

Related US. Application Data Continuation-impart of Ser. No. 266,078,June 26, 1972.

Foreign Application Priority Data July 4, 1971 Israel 37217 US. Cl.172/780; 172/261; 37/D1G. 20; 91/390 Int. Cl. E02S 3/76 Field of Searchl72/4.5, 4, 780, 781, 172/796, 263, 261; 39/DIG. 20, 108; 91/390References Cited UNITED STATES PATENTS Ghent 172/780 X PrimaryExaminerStephen C. Pellegrino Attorney, Agent, or Firm-Flynn andFrishauf [57] ABSTRACT A chassis, adapted to be connected to a farmtractor, supports a blade for grading and levelling; the blade isrotatable with respect to the chassis. The angle of the blade is set bya hydraulic transmission, such as a linear or rotary hydraulic motorcontrolled by a direction controlled valve. Mechanical output from thehydraulic motor is connected to the blade support, typically aturntable, to set the blade angle as determined by the application ofhydraulic pressure. The blade can be fixed in position by engagingtooth-and-detent elements, one of them being respectively fixed to thechassis and the other formed on a turntable which is rotatable with theblade.

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US. Patent Dec. 9, 1975 Sheet 5 of5 3,924,689

HYDRAULICALLY CONTROLLED GRADER-LEVELER ADAPTED FOR TRAILING BY TRACTORSThis application is a continuation-in-part of my earlier applicationSer. No. 266,078, filed June 26, 1972, and claiming priority of Israeliapplication Ser. No. 37,217, of July 4,1971.

This invention relates to an agricultural implement capable of servingboth as a grader and as a leveler and adapted to being attached to afarm tractor by a threepoint hitch.

Graders consist essentially of a heavy, oblong blade of curvedcross-section drawn across uneven ground while fixedly positioned at anangle between zero de- I grees and a right angle to the direction ofmovement.

The implement casts aside those parts of the soil that protrude above acertain predetermined level. Leveler, on the other hand, consistessentially of a blade similar to that used in the grader but drawnacross the ground while at a right angle to the direction of motion,with two rigid aprons or side-shields of strong material on either sideof the blade preventing the soil being carried forward during operationfrom being case aside, thus using it for filling out cavities in thepath of the implement, so that a clearly defined strip of land,substantially more level or even than prior to the leveling operation,is obtained.

Several types of both kinds of implement are known to the art: Thefour-wheeled trailed type has the advantage of being simple andrelatively inexpensive, but it is long, heavy and unwieldy especiallywhen taken together with the tractor. The self-propelled motorgrader-leveler has an adjustable blade that may be positioned in the twomanners appropriate to grading and leveling respectively, the bladebeing provided with side aprons required for a successful gradingaction. It is convenient to use, all operations being readily surveyedand manipulated from the drivers seat, but it representsa largeinvestment in an essentially singlepurpose aggregate.

The three-point hitched grader seeks to overcome the drawbacks of theimplements described above by being shorter, the blade being carried ona wheelless frame and fixed to the tractor by a three-point hitch. Thistype is relatively inexpensive and its action is continuallycontrollable by hand, but it is inconvenient to use and, due to the lackof a support behind the blade, the results of the operation are of a lowquality.

The three-point fixed-blade leveler seeks to overcome some of thedrawbacks of the last named implement by being provided with a supportbehind the blade, and it is also an inexpensive trailer but asinglepurpose implement.

All the aforementioned machines or attached implements are fitted with ablade fixed, at different angles, to the supporting structure in rigidattachment. The blade and/or the structure are therefore liable todamage whenever they encounters an unsurmountable obstruction,especially if this latter is met by the blade in an off-centre placewhereby a bending moment is exerted on the structural parts.

The present invention is designed to further improve the implement by,on the one hand, permitting the setting of the blade angle from thedrivers seat by hydraulic transmission means and, on the other handproviding safety means for letting the blade together with the turntableslip and turn about its axis, upon its encountering excessiveresistance.

It is, therefore, an object of the present invention to provide auniversal leveler and grader which can be trailed by a tractor andconstructed to have, with the tractor, an overall length in the order ofthat of a selfpropelled grader, or less, and in which the blade can beset at a desired angle with respect to the forward motion of theimplement and yet release when an obstacle insurmountable, by theapparatus, is encountered.

SUBJECT MATTER OF THE INVENTION Briefly, the invention relates to acombined leveler and grader attachment for a farm-tractor, to which itcan be secured, for example, by a three-point hitch in which the bladecan be readily set at any angle required for the particular soil workingoperation. In accordance with a feature of the invention, the combinedgrader and leveler comprises a base frame or chassis with means athree-point hitch for trailing attachment to a farm tractor and, at itsrear end, means for supporting it on the ground. A supporting frame maybe attached to the chassis by adjustable means so designed as to enableit to be positioned at different distances from, and different anglesto, said chassis; a normally horizontal ring or turntable is rotatablyattached over the supporting frame to the chassis. A grading andleveling blade is rigidly attached to the said ring or turntable.

The angle of the blade is set by hydraulic transmission means whichcomprises a hydraulic piston motor or actuator of the linear or rotarytype and a direction control valve, for example. The movableforce-exerting portion of the hydraulic motor is suitably connected tothe turntable or the blade in such a manner that when fluid pressure isapplied to one side of the piston or pistons the turntableand-bladecombination is rotated about its axis and brought into the desiredposition. Fluid release means are provided at each side of the pistonhousing and are adjusted to open and release fluid from the respectiveside whenever the fluid pressure exceeds a certain limit correspondingto dangerous load on the blade, thereby permitting the piston or pistonsto move freely and to allow the blade to turn about its axis and therebyto evade the obstruction.

In a preferred embodiment of the invention the release means are in theshape of at least one relief valve, which permits the fluid to flow fromthe piston housing to a collecting vessel.

In another embodiment the hydraulic motor is controlled by asolenoid-operated direction control valve of known design, to beactuated in the required direction by a manual switch, and wherein therelease means consist of one pressure switch each connectedhydraulically to one side of the piston housing, and electrically to oneof the solenoids of the direction control valve, thereby moving thelatter into a position allowing the outflow of fluid from the housing onthe excess-pressure side and to admit fluid to the opposite side. Inaccordance with the invention disclosed in parent application Ser. No.266,078, a positioning and quickrelease mechanism may be provided, whichincludes a tooth with converging surfaces of a shape and size adaptedfor engagement with notches or serrations formed in the turntable. Themechanism is hingedly or slidingly attached to the supporting frame orthe chassis in such a manner that it can be freely moved in or out ofsaid notches. Mechanically, hydraulic, magnetic or other force means acton the tooth, directly or indirectly, to such an extent that the toothholds the blade in position under normal working conditions, but can beforced out of the particular notch it is engaged in by the force exertedby an overload on the blade, for example as the result of an off-centerobstruction, the lat eral force being transmitted from the notch to theinclined contacting surface of the tooth.

Various specific embodiments of the mechanism are shown and described indetail in said application Ser. No. 266,078, the disclosure of which ishereby incorporated by reference.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a side view and part section of a grader-leveler attached to afarm tractor, showing a linear motor as power transmission;

FIG. 2 is a plan and section on line AB-CD of the grader-levelerillustrated in FIG. 1;

FIG. 3 is a longitudinal section of the supporting frame, turntable andblade, showing a different mode of blade-setting mechanism;

FIG. 4 is a plan view of the parts shown in FIG. 3;

FIG. 5 shows a supporting frame, turntable and blade combined with arotary hydraulic motor;

FIG. 6 is a diagrammatic view of a linear hydraulic motor, a directioncontrol valve, the release mechanism and the connecting piping;

FIG. 7 is a diagrammatic view of another hydraulic operating andreleasing mechanism;

FIG. 8 is a diagrammatic view of a combined hydraulic and electricoperating and release mechanism, and

FIG. 9 is a diagram of a rotary hydraulic motor and hydraulic operatingand releasing mechanism.

Referring now to FIGS. 1 and 2 of the drawings a grader-leveler 1 isattached to a farm tractor 2 by a three-point hitch 3, adjusting meansbeing provided in the shape of a hand wheel 4 and a turnbuckle 5 andtheir connection to different holes 6 in the post 7. The implementcomprises a main chassis frame 8 supported at its rear end by twotrailing wheels 9 carried in brackets 10.

A curved grader blade 11 is rigidly fixed, by three struts 13, to aturntable 12 which is shaped in the form of the solid outer ring of aball bearing, also comprising a plurality of balls 14 and on inner ring15 which latter is rigidly connected to a triangular supporting frame16. This supporting frame 16 is fastened at its rear end to the mainchassis frame 8 by two adjustable turnbuckles 17, and at its front endto the post 7 bya ball joint 18 allowing universal angular movement.

The turntable 12, together with the blade 11 can be rotated about theinner ring 15 by means of a linear hydraulic motor which comprises acylinder housing 19, a piston 20 (not visible), and a piston rod 2 1,which latter is pivotally connected at its end to the turntable 12 by avertical pin 22. The cylinder housing, in order to follow the circularmotion of the pin 22 is held pivotally by two vertical pins 24 betweentwo brackets 23 which are integral with the supporting frame 16. Twopositions of the hydraulic motor are shown, one in full lines and theother in dotted lines.

The linear motor is hydraulically connected, by flexible pipes 25, to adirection control valve 26 mounted on the rear part of the farm tractorand operable from the drivers seat. It obtains oil under pressure from ahydraulic pump which is standard equipment with tractors.

FIGS. 3 and 4 show a modification of the turntable, its attachment tothe supporting frame, and the hydraulic angle-setting mechanism. Theturntable 12', in this embodiment, forms the inner ring of a large ballbearing, the outer ring 15 of which is firmly connected to a supportingframe 16'. This is of similar shape as that shown in FIGS. 1 and 2 butpossesses a cross-bar 27 in its front part to the center of whichcross-bar the ball of a ball joint 18 is fastened. The hydraulic linearmotor, in this case, is placed on the inside of the supporting frame 16where it is well protected against damage. Its cylinder housing 19 ispivotally arranged between two bracket bars 23' and held there byvertical pins 24, while the piston rod 21 is at its end pivotallyattached by means of a vertical pin 22 to the turntable 12 at a point onits top surface. Flexible hydraulic connections 25 are fixed to bothends of the piston housing 19. The grader blade 11 is connected to theturntable 12 by three struts 13 similarly as shown in FIGS. 1 and 2.

FIG. 5 shows a different type of turntable and bearing similar to thatdepicted in FIG. 6 of the co-pending patent application Ser. No.266,078. Herein the turntable 12" is in the shape of a solid disc with areinforced outer rim and a reinforced hub, carried by a vertical driveshaft 28, the upper end of which is mechanically connected to a rotaryhydraulic motor 30 positioned on a cross member 31 of the supportingframe 16. The drive shaft passes through a journal bearing 32 fixed in avertical hole in the cross member 31. The blade 11, as previously shown,is fixed to the turntable by struts 13, while the supporting frame 16may be shaped similarly to the supporting frame shown in the aforegoingfigures.

Three different hydraulic operating and release mechanisms are shown inFIGS. 6, 7 and 8 which are designed for a linear motor, but may besimilarly applied to a rotary type hydraulic motor. In these figures thehydraulic motor comprises a cylinder housing 19, a piston 20 and apiston rod 21, the latter transforming the linear movement into rotarymovement of the turntable. Also appearing in all three figures is adirection control valve 26 having a sliding spool 40, shown in itsneutral position in line with the inlet port 41 and spill port 41. Thisspool is so ported that while being moved out of its central position tothe right or left, it supplies oil under pressure to one side of thelinear motor and allows oil from the other side to escape through thespill port, thereby moving the piston and piston rod 21 in the desireddirection into the position required for the grader blade.

Referring now to the diagram in FIG. 6 the spool 40 is held in, andreturned to, its central position by two springs 42 and 42' which arearranged within the closed valve housing 26. This housing, in additionto the inlet and spill ports 41 and 41, possesses two supply and returnports 43, 43 situated opposite the above ports, and two ports each 44and 44 situated near the closed outer ends, which ports serve to admitfluid under pressure to the end faces of the spool with the aim ofmoving it out of its central position into either of the operatingpositions. A hand-operated four-way valve 45, which obtains fluid underpressure from the central supply, directs it to the right or left sideof the control valve 26 through pipes 48 and 48' respectively, andempties the opposite end into a collecting vessel 54. The supply andreturn ports of the valve are connected to one side each of the cylinderhousing by piping 47 and 47.-From each of these pipes a return pipe 49,49'

enters the valve housing through one of the ports 44, 44, and each pipeis normally closed by a nonreturn sequence valve 50, 50', of knowndesign, which are adjusted to open when the pressure on .the respectiveside of the cylinder housing exceeds a predetermined pressure. Owing tothe different face areas onboth sides of the piston, the two sequencevalves will be set to different pressure.

The operation of the mechanism is as follows: Upon turning the handle ofthe fourway valve 45, for instance clockwise, fluid enters the right endof the control valve housing through pipe 48 and pushes the slidingspool 40 to the left until the right-hand spool ports coincide with, andthus connect, ports 41 and 41' with ports 43 and 43"of the valve housing26 respectively. Thus oil is pressed into the left hand side of thecylinder housing, pushing the piston 20 and the piston rod 21 to theright until the desired blade position is reached. Then the four-wayvalve 45 is closed and the implement can start on its way. If the bladenow encounters excessive resistance at one of its ends, which acts as aforce transmitted via the turntable and the piston rod to the piston,pressure is built up in the cylinder housing at one end which eventuallyopens the respective se quence valve (50, 50') on this side. Assumingthat valve 50 has opened then fluid will escape into the left side ofthe 'control valve casing and push the spool to the right, thuspressingfluid into the right'end of the cylinder housing and drainingthe left end. Thispushes the piston and piston rod further to the leftrelieving the pressure on the blade. The pressure drop on the left sidewhich is being drained lets the sequence valve 50 close and the bladecan now be returned by manual operation of the fourway valve.

The diagram of FIG. 7 shows a'hand-operated direction control valve 26with its operating-handle 51 on its left side. It can readily be seenthat by moving the spool 40 to the right for instance, the piston 20 isalso pushed to the right and vice versa. The overload release mechanismin this case consists of a relief valve 500 placed between the two pipes47 and 47 connecting the hydraulic motor to the control valve 26.Furthermore the pipes 47 and 47 are protected against shortcircuiting bytwo check valves 52, 52', which allow fluid to pass to and through therelief valve but not between the two pipe lines. Two more check valves,53, 53', areconnected to pipe lines 47, 47' respectively and dip intofluid in the collection vessel 54 thus serving as anticavitation valvesfor returning fluid into the enlarging space at the other end of thecylinder.

FIG. 8 shows a combined electric-hydraulic mechanism wherein themovement of the valve spool is induced by two solenoid windings 60 and60' and two cores 61 and 61 rigidly fixed to one end each of the spool.

Two electric pressure switches 63, 63 are hydraulically connected to oneside each of the cylinder housing of the hydraulic motor andelectrically connected to the respective solenoid windings 60, 60. Ahand operated switch 62, 62' transmits current to either solenoidwinding which pulls the core and the spool in its direction.

The release mechanism operates by excess pressure, for example on theleft side of the piston closing the left hand pressure switch 63 whichthen causes current to flow to the left solenoid windings 60. This drawsthe core 61 and the spool 40 to the left, whereby the left hand ports inthe spool allow fluid. to enter the right 6 side of the cylinder, theleft side being drained at the same time. As soon as the overload ceasesto exert pressure the pressure switch, contacts open and the spool isreturned to its central position by springs not shown in the diagram.This enables the blade to be reset by operating one of the switches 62,62.

The rotary motor.30 shownin FIG. 9 is actuated by a hand-operateddirection control valve 26, wherein the spool 40 can be moved out of itsneutral position to the right or to the left by a handle 51. Pipe lines47 and 47' connect the control valve withthe rotary motor which rotatesin either direction in accordance with the position ,of the valve spool.One relief valve (50, 50') each branches off from the connecting pipes(47, 47 and leads into a collecting vessel 54. In parallel to the reliefvalves two check valves 52 and 52' are installed, acting asanti-cavitation valves.

The operation is'similar to'that of the arrangement shown in FIG. 7 withthe difference that two relief valves are installed instead of one unitonly. The check valve outlets end below the liquid surface in thecollecting ,vessel, so as to enable them to act as anti-cavitationvalves, returning liquid to the opposite side of the pistons wheneverone of the relief valves opens and empties one side.

In essence, the present invention permits setting of the blade atpractically any desired angle, from trans: verse to the direction ofmovement of the implement across a field to practically in line withthis movement. The piston motor is provided with an overload reliefvalve, If pressure on the blade, and hence on the'operating rod shouldexceed a certain safe value, the setting of the blade releases and theblade can freely swing into line with the direction of travel of theimplement, thus clearing the obstance, and avoiding damage to theblade,-the implement, .and the tractor-implement assembly. The driver ofthe tractor merely. need re-actuate the hydraulic motor, which firstcloses the relief valve, and then repositions the blade to its propersetting, to continue the work intended for the implement.

Various changes and modifications may be made within the scope of theinventive concept, and embodiments shown in any one of the figures, ordescribed in connection therewith, may be used with any embodiment shownor described, as desirable and appropriate.

lclaim:

1. Off-center overload protected grader-leveler apparatus comprising achassis having means at its forward end for trailing attachment to afarm tractor and, at its rear end, means for supporting the chassisabove ground, said tractor having a hydraulic system including acollecting vessel;

a blade support means, and means rotatably securing the blade supportmeans to the chassis to enable the support means to swing about anessentially vertical axis to permit selective positioning of the supportmeans with respect to said chassis;

a grading and leveling blade supported by the support means;

blade positioning means, quick release means including a hydraulicpiston motor having a cylinder;

means connecting the movable force transmitting portion of the hydraulicmotor to the blade support means;

and means controlling application of pressurized fluid, selectively, toa side of the piston of the motor in order to rotate the blade supportmeans,

and hence the blade about its axis and bring it into a desired position;wherein, to avoid blade damage upon encountering an off-center overload,the improvement comprises controlled quick fluid release means to openand rclease fluid from the respective overloaded side of the piston intothe collecting vessel whenever the fluid pressure on the piston exceedsa limit defined by an excessive off-center load on the blade, release offluid permitting the piston to move freely and to allow 'the bladesupport means with the blade'attached thereto to turn freely about itsaxis and thereby to prevent damage to the apparatus. 2. Apparatusaccording to claim 1, wherei'nthe blade support means comprises anormally horizontal turntable, the blade being secured to the turntable.3. Apparatus according to claim 1, wherein the release means on eachside of the cylinder comprises a relief valve which permits, at excesspressure, fluid to be drained from its side andto flow into thecollecting vessel; and an anti-cavitation valve is connected in parallelwith the relief valve and permitting fluid from the collecting vessel toenter the other side of the cylinder as the piston is moving underinfluence of an overload.

4. Apparatus according to claim 3, wherein the release means compriseone single relief valve connected to both sides of the piston housing;one check-valve each connecting the sides of the cylinder to the singlerelief valve;

and two anti-cavitation valves provided between the the requireddirection by selective energization of the solenoid controlled valve;

and wherein the release means comprises a pressure switch, each,connected hydraulically to one side of the cylinder. and electrically toone of the solenoids of the direction control valve, thereby moving thelatter into a position allowing the outflow of fluid from the housing onthe excess-pressure side and to admit fluid to the opposite side.

6. Apparatus according to claim 1, wherein the means for supporting thechassis above ground comprises at least one wheel attached to the rearpart of the chassis.

7. Apparatus according to claim 1, wherein a plurality of wheels areprovided, said wheels being directionally self-adjusting caster wheels.

8. Apparatus according to claim 1, wherein the means for supporting thechassis above ground level comprises at least one roller attached to therear part of the chassis.

9. Apparatus according to claim 1, wherein the blade support meanscomprises a turntable.

10. Apparatus according to claim 1, wherein the controlled quick fluidrelease means comprises a relief valve, each, located at either side ofthe piston to release pressurized fluid from a respective side thereof.

11. Apparatus according to claim 1, wherein the controlled quick fluidrelease means comprises a relief valve which permits, at excesspressure, fluid to be drained from its side and to flow into thecollecting vessel;

and an anti-cavitation valve connected in parallel with the relief valveand permitting fluid from the collecting vessel to enter the other sideof the cylinder as the piston is moving under influence of an overload.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3 924,689

DATED 1 December 9, 1975 INVENTOR(S) Gedalyahu MANOR It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Title Page, Item 30, change to read;

July 4, 1971 Israel 37217 March 5, 1973 Israel 4l68l Signed and Scaledthis Twenty-sixth Day of October 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Office" Commissioner oj'Palenrsand Trademarks UNITED STATES PATENT OFFICE @ETEFEQATFE 9F CQECTIGNPatent No. 2 9 3? Dated December 9, 1 975 Inventor s) Gedalyahu MANOR Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 6, line 61 (claim 1 delete "quick release means".

gigncd and fizzled this twenty-third 3) 0f March 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissinm'r uj'larenrsand Trademarks

1. Off-center overload protected grader-leveler apparatus comprising a chassis having means at its forward end for trailing attachment to a farm tractor and, at its rear end, means for supporting the chassis above ground, said tractor having a hydraulic system including a collecting vessel; a blade support means, and means rotatably securing the blade support means to the chassis to enable the support means to swing about an essentially vertical axis to permit selective positioning of the support means with respect to said chassis; a grading and leveling blade supported by the support means; blade positioning means, quick release means including a hydraulic piston motor having a cylinder; means connecting the movable force transmitting portion of the hydraulic motor to the blade support means; and means controlling application of pressurized fluid, selectively, to a side of the piston of the motor in order to rotate the blade support means, and hence the blade about its axis and bring it into a desired position; wherein, to avoid blade damage upon encountering an off-center overload, the improvement comprises controlled quick fluid release means to open and release fluid from the respective overloaded side of the piston into the collecting vessel whenever the fluid pressure on the piston exceeds a limit defined by an excessive off-center load on the blade, release of fluid permitting the piston to move freely and to allow the blade support means with the blade attached thereto to turn freely about its axis and thereby to prevent damage to the apparatus.
 2. Apparatus according to claim 1, wherein the blade support means comprises a normally horizontal turntable, the blade being secured to the turntable.
 3. Apparatus according to claim 1, wherein the release means on each side of the cylinder comprises a relief valve which permits, at excess pressure, fluid to be drained from its side and to flow into the collecting vessel; and an anti-cavitation valve is connected in parallel with the relief valve and permitting fluid from the collecting vessel to enter the other side of the cylinder as the piston is moving under influence of an overload.
 4. Apparatus according to claim 3, wherein the release means comprise one single relief valve connected to both sides of the piston housing; one check-valve each connecting the sides of the cylinder to the single relief valve; and two anti-cavitation valves provided between the two sides of the pistOn housing and the collecting vessel.
 5. Apparatus according to claim 1, comprising a solenoid-operated direction control valve connected to control the hydraulic motor and to actuate the motor in the required direction by selective energization of the solenoid controlled valve; and wherein the release means comprises a pressure switch, each, connected hydraulically to one side of the cylinder, and electrically to one of the solenoids of the direction control valve, thereby moving the latter into a position allowing the outflow of fluid from the housing on the excess-pressure side and to admit fluid to the opposite side.
 6. Apparatus according to claim 1, wherein the means for supporting the chassis above ground comprises at least one wheel attached to the rear part of the chassis.
 7. Apparatus according to claim 1, wherein a plurality of wheels are provided, said wheels being directionally self-adjusting caster wheels.
 8. Apparatus according to claim 1, wherein the means for supporting the chassis above ground level comprises at least one roller attached to the rear part of the chassis.
 9. Apparatus according to claim 1, wherein the blade support means comprises a turntable.
 10. Apparatus according to claim 1, wherein the controlled quick fluid release means comprises a relief valve, each, located at either side of the piston to release pressurized fluid from a respective side thereof.
 11. Apparatus according to claim 1, wherein the controlled quick fluid release means comprises a relief valve which permits, at excess pressure, fluid to be drained from its side and to flow into the collecting vessel; and an anti-cavitation valve connected in parallel with the relief valve and permitting fluid from the collecting vessel to enter the other side of the cylinder as the piston is moving under influence of an overload. 